This invention relates to the automatic rounding or deburring of sharp edges on rib sides through shot impact.
A number of high strength, light weight, duct-like structures for airplane, spacecraft or other applications are manufactured by machining or chemical milling a pattern of pockets surrounded by upstanding ribs over the outer surface of a metal workpiece which is a surface of revolution. For an optimum combination of high strength and light weight, the ribs often have non-uniform thicknesses in selected regions. When produced, these ribs often have sharp edges along their sides, along lines substantially parallel to the inner skin surface, especially along boundaries of different rib thickness regions.
It is necessary that these sharp edges be deburred; that is, removed or rounded off because the high stress concentrations at the sharp edges can cause stress cracking and to eliminate the hazard of cutting fingers and hands while handling the component during the manufacturing process. In the past, these sharp edges have been manually deburred, using buffers, scrapers or grinders. These methods are labor intensive, require considerable skill, often require careful hand work in corners or other inaccessible areas and can damage parts, requiring careful inspection and rework, often resulting in unacceptable scrap rates.
Impacting metal surfaces with high velocity particles, such as sand particles or steel or ceramic shot, has long been used to clean the surface of contaminants or scale and flash. Shot peening with small metal balls is widely used to work harden metal surfaces such as turbine blades by increasing surface stresses in the workpiece. Dropping or propelling metal shot against metal surfaces can also smooth and polish the surfaces.
One problem with shot peening has been the tendency to damage edges when peening a workpiece. Neal, et al., in U.S. Pat. No. 4,426,867, describe a method and apparatus for peening airfoils and thin edged workpieces which avoids impact at edges to avoid damage, teaching that the shot streamline should be kept away from edges.
A number of patents, such as U.S. Pat. No. 4,067,240 to Straub, show apparatus for shot peening in which shot from a hopper below a workpiece is lifted by a conveyor or elevator to a hopper having a bottom opening through which the shot falls, impacting the workpiece and returning to the hopper. These prior devices are effective in uniformly peening the entire surface of a workpiece, but are incapable of peening only selected areas without impacting adjacent, more damage sensitive, areas.
In some cases, as described in U.S. Pat. No. 4,228,671 by Skeen, the entire workpiece is first peened with large shot to improve surface strength, but then must be peened with finer shot to provide an acceptably smooth appearance.
In order to uniformly peen round workpieces, such as gears, shafts, or the like, it is necessary to rotate the workpiece. Young, et al., in U.S. Pat. No. 2,542,955, attach paddlewheels to the ends of such parts and allow the shot to strike the paddlewheel in a manner causing the workpiece to rotate. This is a somewhat cumbersome device and requires the shot stream to cover a large area, preventing careful direction of the stream against selected areas. Other complex moving devices have been developed for moving long or large workpieces past nozzles which spray shot entrained in high velocity air streams at the workpieces. Typical of these is the apparatus described in U.S. Pat. No. 2,982,007 by Fuchs, et al. These devices tend to be large, complex, require air streams to carry shot and are not easily adjustable to handle different workpieces.
Compton, in U.S. Pat. No. 3,485,074, describes an apparatus for cleaning and deburring small holes in metal structures by manually directing a small, high velocity air stream with small glass beads entrained therein against the surface to be treated. This apparatus is not capable of automatically treating selected areas of large structures and requires a complex air stream generating means and bead entrainment means.
Thus, there is a continuing need for improved methods and apparatus for deburring or rounding sharp edges on ribs of large duct-like structures. While round media are used in several known techniques for increasing residual stresses in metal surface and for cleaning or smoothing such surfaces, nothing in the industry suggests any way that round media could be adapted to overcome that problem.